© 2020 Elsevier B.V.This study presents an investigation of colorimetric sensing of mercury ions in aqueous medium using chlorophyll (Chl) functionalized green synthesized silver nanoparticles (AgNPs). Chl-AgNPs showed a high visually detectable color change for the selective detection of mercury ions, while the unmodified green synthesized silver nanoparticles did not show any remarkable color change. The surface plasmon resonance absorption band of Chl-AgNPs was obtained at 440 nm area and decreased with a blue shift by the addition of mercury ion. Furthermore, an increase was detected at the absorption band of 290 nm attributed to the blank silver ions. The possible mechanism relies on the redox reaction between the silver nanoparticles and mercury ions. The detection limit of the Chl-AgNPs was obtained as 2.7 µM and 60 µM by the UV-vis spectrophotometer and the naked eye, respectively. The response of Chl-AgNPs towards the mercury ion was excellent over other metal ions. The functionalization of green silver nanoparticles using chlorophyll pigment provided colorimetric detection advantage by the naked eye. The proposed method was successfully applied to river water samples spiked with known concentrations of mercury ions at the recovery values between 94 to 97%.